This invention relates in general to apparatus for sensing relative displacement, and in particular instances to apparatus for measuring strain in an object.
Measurement of strain (the change in length of an object in some direction per unit undistorted length) in specimens and objects may be carried out either directly or indirectly. Some of the approaches used for direct strain measurements include the use of bonded wire strain gauges (in which a grid of strain sensitive wire is cemented to a specimen so that a change in the length of the grid due to strains in that specimen changes the resistance of the wire which can then be measured), mechanical strain gauges (in which optical or mechanical lever systems are employed to multiply the strain which may then be read from a suitable scale), magnetic strain gauges (which include magnetic circuits having air gaps which, when varied as a result of a strain in the specimens, varies the permeance of the circuits to provide an indication of the strains produced), semiconductor strain gauges (in which the resistance of a piezoelectric material varies with applied stress and resulting strain in the material), capacitance strain gauges (in which a variation of capacitance caused by variation in the separation of elements due to strain in the specimen, can be measured to provide a reading of the strain), and field-based strain sensors (in which a flexible substrate includes an electric field-producing element and one or more electric field detecting elements for determining position of the detecting elements relative to the field-producing element to thereby provide a measure of relative movement and thus a reading of the strain in an object to which the substrate is attached). Other direct strain measuring devices include acoustic strain gauges, brittle lacquer coatings and photogrids.
Approaches for indirectly measuring strain in a specimen include the use of displacement pick-up devices, velocity pick-up devices, and acceleration detection devices.
A disadvantage of some of the conventional approaches to measuring strain is that the devices employed are oftentimes difficult to attach to or use with a specimen whose strain is to be measured. Also, such devices are typically difficult and costly to manufacture. Finally, because of the intrinsically high axial rigidity of many of such devices, it requires high quality bonding of the devices to the specimen to prevent detachment due to failure of the bond and this, in turn, requires time-consuming and careful preparation of the specimen for bonding.
The field-based strain sensor briefly mentioned above obviates and overcomes some of these problems and can be made very compactly and inexpensively (see U.S. Pat. No. 4,964,306). However, for some applications, even more precise, strain transducers would be required or at least desirable.